how to check a filesystem in Ubuntu 16.04?

To check the file system on your Ubuntu partition...

• boot to the GRUB menu
• choose Advanced Options
• choose Recovery mode
• choose Root access
• at the # prompt, type sudo fsck -f /
• repeat the fsck command if there were errors
• type reboot

If for some reason you can't do the above...

• boot to a Ubuntu Live DVD/USB
• start gparted and determine which /dev/sdaX is your Ubuntu EXT4 partition
• quit gparted
• open a terminal window
• type sudo fsck -f /dev/sdaX # replacing X with the number you found earlier
• repeat the fsck command if there were errors
• type reboot